While commercially still in its infancy, optical switching (i.e., switching in the optical domain) is well known in the switching art. A commonly-used optical switching element is the Y-branch switching element, so called because of its 1.times.2 topology: it has one optical-signal input port and two optical-signal output ports, or alteratively has two optical-signal input ports and one optical-signal output port. The Y-branch switching element is commonly implemented in a lithium niobate (LiNbO.sub.3) substrate, which has the desirable property that when voltage is applied to it, its index of refraction changes. This principle is used for control of the Y-branch switching element: light entering the single input port is channeled to the one of the two output ports that has the higher index of refraction, or alternatively light from the one of the two input ports that has the higher index of refraction is channeled to the single output port.
A problem with lithium niobate-implemented Y-branch switching elements is that chip-fabrication techniques presently use too great a length of substrate--on the order of one centimeter--to implement each Y-branch switching element, and hence consume a lot of valuable space, or "real estate", on the fabrication wafer. This length can be decreased, but there is a tradeoff: the shorter the device is, the higher is the control voltage required to switch it. And higher control voltages are undesirable because they require a greater control voltage swing, resulting in higher power requirements and slower device operation.
To form a switching fabric, switching elements are normally not used individually in isolation, but rather arrays of the elements are interconnected. To be commercially practical, a switching fabric of some basic minimum size, for example, an 8-input-by-8-output switching fabric, has to be integrated into a single package. Hence, multiple switching elements must be interconnected on a single chip via waveguides. In lithium niobate devices, the waveguides are generally implemented by diffusing titanium into the substrate. To achieve practical interconnection of the switching elements, the waveguides are, for the most part, not straight lines, but rather have bends. Here, another set of problems with lithium niobate-implemented switching elements comes into play: the smaller the radius of the bends in the waveguides is, the greater is the loss of optical power in the waveguides. The radius of bends in the waveguides can be reduced, but only at the cost of making the waveguides longer. And longer waveguides result in greater spacing between interconnected switching elements, thereby again consuming valuable "real estate" on the fabrication wafer.
Hence, there are practical limits on the minimum size of Y-branch switching devices and on their spacing in a switching-fabric array. But there are also practical limits on the maximum size of the fabrication wafers on which the switching-fabric arrays of the devices can be implemented. Also, there are economic benefits in integrating as much functionality as possible on available wafer sizes. Therefore, it is highly desirable to limit the number of switching elements, and the number and length of the interconnections between these switching elements, that make up the basic-size switching fabric.
Numerous switching-fabric architectures are known to the art. Included among them are the Benes network, the dilated Benes network, and the active-splitter/active combiner topology. Each network architecture has its own advantages and disadvantages. For example, a Benes network is very efficient in terms of the number of switching elements that are required to implement it. Also, it is a rectangular network that requires a minimum number of switching elements in each column of the network, which tends to minimize the complexity of interconnections between the columns. But a Benes network also requires relatively many columns of switching elements to implement it, which requires a relatively large number of interconnections and consumes relatively large amounts of fabrication-wafer real estate.
A dilated Benes network has many of the same attributes of the Benes network. Its advantage is very low cross-talk between paths through the network. But the advantage is achieved at the cost of more-than-doubling the number of switching elements that are needed to implement it, relative to the simple Benes network. Worse yet, the number of columns of the switching elements is increased relative to the simple Benes network.
In contrast, an active-splitter/active-combiner network is very efficient in terms of the number of columns of switching elements that are required to implement it. However, it requires the height of the columns (the number of switching devices per column) to increase geometrically in the middle stages of the switching-fabric array for every arithmetic increase in the size of the switching fabric, which leads to complex routing and rapidly increases the numbers of required interconnections.